T1-weighted multiecho magnetic resonance imaging

ABSTRACT

The invention relates to a method for magnetic resonance imaging of at least a portion of a body placed in a stationary and substantially homogeneous main magnetic field. The body is subjected to a sequence of RF and magnetic field gradient pulses during an interval TSE, thereby generating a plurality of spin echo signals, which are measured and processed for reconstruction of an image. Thereafter, during an interval TDRV, an additional spin echo is generated by subjecting the body to at least one further refocusing RF pulse and/or magnetic field gradient pulse, and a RF drive pulse (βX) is irradiated at the time of this additional spin echo. In order to provide a fast and reliable method for T 1 -weighted imaging, which gives a high T 1  contrast and also a sufficient signal-to-noise ratio, the phase of the RF drive pulse (βX) is selected such that nuclear magnetization at the time of the additional spin echo is transformed into negative longitudinal magnetization. The sequence is repeated beginning with another sequence of RF and magnetic field gradient pulses after a recovery period TREC.

The invention relates to a method for magnetic resonance imaging of atleast a portion of a body placed in a stationary and substantiallyhomogeneous main magnetic field, the method comprising the followingsteps:

-   -   a) subjecting said portion of a body to a sequence of at least        one RF pulse and at least one magnetic field gradient pulse,        thereby generating one or more spin echo signals in said        portion;    -   b) measuring said spin echo signals for reconstructing an image        from said signals;    -   c) generating an additional spin echo by subjecting said portion        to at least one further refocusing RF pulse and/or magnetic        field gradient pulse;    -   d) subjecting said portion to a RF drive pulse at the time of        said additional spin echo;    -   e) repeating steps a) to d) with the pulse sequence of step a)        beginning after a recovery period.

Furthermore, the invention relates to a device for magnetic resonanceimaging for carrying out this method.

In magnetic resonance imaging (MRI), pulse sequences consisting of RFand magnetic field gradient pulses are applied to an object (a patient)to generate magnetic resonance signals, which are scanned in order toobtain information therefrom and to reconstruct images of the object.Since its initial development, the number of clinical relevant fields ofapplication of MRI has grown enormously. MRI can be applied to almostevery part of the body, and it can be used to obtain information about anumber of important functions of the human body. The pulse sequencewhich is applied during a MRI scan determines completely thecharacteristics of the reconstructed images, such as location andorientation in the object, dimensions, resolution, signal-to-noiseratio, contrast, sensitivity for movements, etcetera. An operator of aMRI device has to choose the appropriate sequence and has to adjust andoptimize its parameters for the respective application.

T₂-contrast enhanced spin echo sequences are the most widely used scansin clinical application because they provide exquisite soft tissuecontrast. However, their utility is limited by their long acquisitiontime. This is why fast T₂-contrast enhanced sequences have been theobject of recent developments. Known techniques, which are capable ofproducing T₂-weighted images within a few seconds, are spin echosequences like the so-called RARE (rapid acquisition by repeated echoes)and TSE (turbo spin echo) sequences. The known spin echo sequencesusually consist of an initial contrast preparation period and asubsequent data acquisition period. During the first period thelongitudinal magnetization is prepared according to the desired contrastwhile many phase-encoded echoes are generated and acquired during thesecond period to form the image.

A T₂-contrast enhanced MRI method which is particularly useful forimaging of tissue with a high T₂/T₁ ratio is known from the U.S. Pat.No. 6,219,571 B1. This technique uses a so-called driven equilibriumFourier transform (DEFT) sequence, which enhances signal strengthwithout waiting for fall T₁ recovery. DEFT imaging provides particularlyhigh contrast between tissues with different values of T₂ whilemaintaining a high signal to noise ratio. The typical DEFT pulsesequence begins with a slice-selective RF excitation pulse (α_(X))followed by a plurality of 180°_(Y) refocusing pulses resulting in phaseencoded spin echoes, which are used for imaging. At the last spin echo,a “driven equilibrium” (or simply “drive”) pulse (−α_(X)) transforms theremaining transverse magnetization into positive longitudinalmagnetization. An optional gradient pulse at the end of the sequencemight help to spoil residual transverse magnetization. This pulsesequence is then repeated after a recovery period which is shorter thanthe T₁ of the examined tissue.

It is well known that also the generation of high quality T₁-contrastenhanced images is essential for many MRI applications. This isparticularly valid for the examination of the human brain. For example,the significant difference between the longitudinal relaxation times ofwhite matter tissue and cerebra spinal fluid can be utilized to produceimages with high contrast between these two materials. The main drawbackof the well-known inversion recovery (IR) techniques, which areroutinely employed for T₁-weighted imaging, is again their comparativelylong acquisition time. Adequate and efficient T₁-weighted imaging isespecially challenging at high magnetic fields of 3 Tesla or more, as asignificant increase of T₁ is then observed.

Therefore it is readily appreciated that there is a need for MRI methodswhich enable a fast T₁-contrast enhanced imaging. It is consequently theprimary object of the present invention to provide a fast and reliablemethod for T₁-weighted imaging, which gives a high T₁ contrast and alsoa sufficient signal-to-noise ratio.

In accordance with the present invention, a method for magneticresonance imaging of the type specified above is disclosed, wherein theaforementioned object is achieved by selecting the phase of said RFdrive pulse such that nuclear magnetization at the time of saidadditional spin echo is transformed into negative longitudinalmagnetization.

The present invention enables to perform fast tomographic scanning withsignificantly enhanced T₁ contrast. While the method of the invention isparticularly valuable for TSE (turbo spin echo) imaging, it can also beapplied to any spin echo based imaging sequence. The structure of thesequence of the invention is similar to the above-described DEFTsequence. But the essential difference is that the remainingmagnetization after the spin echo data acquisition (i.e. in most casesmainly transverse magnetization) is transformed into negativelongitudinal magnetization. It turns out that this difference causes thedesired T₁-contrast enhancement instead of the T₂-enhancement which isobtained with the known DEFT method.

By generating the additional spin echo in step c) after the imagingreadout in step b) all transverse magnetization is refocused. As alreadydescribed above, the magnetization is then transformed into negativelongitudinal magnetization by applying the RF drive pulse at this echoin step d). If the excitation angle of the spin echo sequence in step a)is not equal to 90°, the RF drive pulse will generally also not be equalto 90° in order to achieve optimal contrast enhancement. In casesubstantial T₁ decay takes place during the readout period in step b),the flip angle of the drive pulse will generally be larger than 90°,since an optimal signal will be obtained by generating as much negativelongitudinal magnetization as possible. By allowing T₁ decay after theRF drive pulse in the recovery period of step e) before the nextexcitation begins in step a), a T₁-contrast enhancement is obtainedsimilar to the known inversion recovery technique. In contrast to IR theinversion is not performed by a 180° pulse, but by the RF drive pulsewhich is irradiated when the transverse magnetization is coherent(refocused). This mechanism of the invention results in a very compactsequence, allowing for short repetition times and hence high T₁contrast. Furthermore, the method of the present invention is simple,robust, and fast and can easily be implemented on any magnetic resonanceimaging apparatus in clinical operation.

With the method of the invention it is advantageous if the sequence ofstep a) begins with a substantially 90° RF pulse. A maximum of initialmagnetization can be obtained in this way. In case only little T₁ decaytakes place during the readout period of step b), it is practical if thedrive pulse of step d) is a substantially 90° RF pulse as well, sincethe nuclear magnetization, which is mainly transverse magnetization,will be almost completely transformed into negative longitudinalmagnetization in this way. In a practical implementation of theinvention, the sequence of step a) begins with an RF pulse, which hassubstantially the same flip angle as the drive pulse of step d), boththe initial RF pulse of step a) and the drive pulse of step d) alsohaving substantially the same RF phase. Again in case only little T₁decay takes place during the readout period of step b), both the initialRF pulse of step a) and the drive pulse of step d) will preferably havea flip angle of about 90°.

Experiments and numerical simulations show that with the method of theinvention it is advantageous if the pulse sequence of step a) is a turbospin echo (TSE) sequence beginning with a substantially 90° X-pulsefollowed by a series of 180° Y-pulses, wherein the drive pulse is againa substantially 90°X-pulse. The 180° refocusing pulses can be replacedby so-called composite pulses, which also result in a 180° tip of themagnetization, in order to make the method still less sensitive tovariations in B₀ (i.e. the main magnetic field) and B₁ (i.e. the RFfield). For optimizing the sequence, it is further possible toincorporate a flip angle sweep for the refocusing pulses.

The method for magnetic resonance imaging of the invention is alsowell-suited to be used with other known spin echo based imagingsequences in step a), such as a RARE (rapid acquisition by repeatedechoes) sequence, an EPI (echo planar imaging) sequence, or a GRASE(gradient and spin echo) sequence.

With the method of the invention it turns out that the T₁ contrastbetween different materials can in most cases be optimized if theduration of the recovery period is selected such that it is larger thanthe shortest T₁. But in general the optimization of the parameters ofthe sequence in terms of contrast, speed, and signal-to-noise isstrongly dependend on the ratios of the different T₁ values and thesignal strength of the different materials under examination.

It is easily possible to incorporate the method of the present inventionin a dedicated device for magnetic resonance imaging of a body placed ina stationary and substantially homogeneous main magnetic field. Such aMRI scanner comprises means for establishing the main magnetic field,means for generating gradient magnetic fields superimposed upon the mainmagnetic field, means for radiating RF pulses towards the body, controlmeans for controlling the generation of the gradient magnetic fields andthe RF pulses, means for receiving and sampling magnetic resonancesignals generated by sequences of RF pulses and switched gradientmagnetic fields, and reconstruction means for forming an image from saidsignal samples. In accordance with the invention, the control means,which is usually a microcomputer with a memory and a program control,comprises a programming with a description of an imaging procedureaccording to the above-described method of the invention. Forinteractive scanning, an input means may be provided, which enables anoperator of the device to adjust the flip angle and the RF phase of thedrive pulse of step d) as well as the duration of the recovery period ofstep e) while the imaging procedure is in progress. An appropriate userinterface will then allow for an interactive optimization of the desiredcontrast.

The following drawings disclose preferred embodiments of the presentinvention. It should be understood, however, that the drawings aredesigned for the purpose of illustration only and not as a definition ofthe limits of the invention.

IN THE DRAWINGS

FIG. 1 shows a diagram of a pulse sequence in accordance with thepresent invention;

FIG. 2 shows steady-state magnetizations during the sequence of FIG. 1;

FIG. 3 shows an embodiment of a MRI scanner according to the invention.

A sequence design in accordance with the present invention is depictedin FIG. 1. The sequence begins with a—typically slice selective—RFexcitation pulse α_(X). This pulse tips the magnetization through anangle α about the x-axis. In most spin echo sequences, the first pulsewill have an excitation angle of 90°. A series of 180°_(Y) refocusingpulses following the first pulse result in a plurality of spin echoeswhich are not represented in the figure. The 180°_(Y) pulses and thespin echoes are separated by a time interval ES (echo spacing). The spinechoes are measured during a readout interval TSE. These registeredsignals are subsequently used for imaging. Images can for example bereconstructed by subjecting the acquired data to a well-known 2DFT-procedure. During the interval TSE, the depicted sequence correspondsto a usual turbo spin echo sequence. But after the last pulse of the TSEinterval, an additional spin echo is generated by a further 180°_(Y)pulse. At this spin echo, an β_(X) drive pulse tips the remainingcoherent magnetization to the negative longitudinal axis. The TSEsequence is thus extended by an additional interval TDRV, after whichthe transverse magnetization has been transformed into negativelongitudinal magnetization. A subsequent gradient pulse, which is notrepresented in the figure, optionally helps to spoil residual transversemagnetization. Another TSE sequence begins with a α_(X) excitation pulseafter a recovery period TREC, during which T₁-relaxation is allowed totake place. The repetition time of the sequence is TR. α and β will beequal, if no significant T₁ decay takes place during the TSE readout.

The temporal development of the steady state magnetization during thesequence of FIG. 1 is shown in FIG. 2. The figure shows magnetizationsM₁ and M₂ of two different materials (for example white matter andcerebra spinal fluid of the human brain), wherein the longitudinalrelaxation time of the first material is much shorter than therelaxation time of the second material. During the first part of thesequence, which is composed of the intervals TSE and TDRV, themagnetizations M₁ and M₂ are in the transverse x,y-plane, wheretransverse relaxation takes place. During the recovery period TREC, themagnetizations are directed along the longitudinal z-axis. As depictedin the figure, longitudinal relaxation with two different rates takesplace during this period. By allowing T₁ relaxation during TREC afterthe drive pulse until the next excitation, a T₁ contrast enhancement isobtained similar to the known IR technique. The inversion is notperformed by a 180° pulse, but by the β_(X) drive pulse at a point intime when the transverse magnetization is coherent. Therefore, themethod of the invention can also be characterized as “coherent inversionrecovery”. The mechanism of the invention allows for a compact sequencewith short TR and high T₁ contrast. The different amplitudes of the M₁and M₂ magnetizations during the data acquisition interval TSE canclearly be seen in FIG. 2. This difference results from the fast (M₁)and slow (M₂) relaxation during TREC.

In FIG. 3 a magnetic resonance imaging device 1 is diagrammaticallyshown. The apparatus 1 comprises a set of main magnetic coils 2 forgenerating a stationary and homogeneous main magnetic field and threesets of gradient coils 3, 4 and 5 for superimposing additional magneticfields with controllable strength and having a gradient in a selecteddirection. Conventionally, the direction of the main magnetic field islabelled the z-direction, the two directions perpendicular thereto thex- and y-directions. The gradient coils are energized via a power supply11. The apparatus 1 further comprises a radiation emitter 6, an antennaor coil, for emitting radio frequency (RF) pulses to a body 7, theradiation emitter 6 being coupled to a modulator 8 for generating andmodulating the RF pulses. Also provided is a receiver for receiving theMR-signals, the receiver can be identical to the emitter 6 or beseparate. If the emitter and receiver are physically the same antenna orcoil as shown in FIG. 3, a send-receive switch 9 is arranged to separatethe received signals from the pulses to be emitted. The receivedMR-signals are input to a demodulator 10. The modulator 8, the emitter 6and the power supply 11 for the gradient coils 3, 4 and 5 are controlledby a control system 12 to generate the above-described sequence of RFpulses and a corresponding sequence of magnetic field gradient pulses.The control system is usually a microcomputer with a memory and aprogram control. For the practical implementation of the invention itcomprises a programming with a description of an imaging procedureaccording to the above-described method. The demodulator 10 is coupledto a data processing unit 14, for example a computer, for transformationof the received echo signals into an image that can be made visible, forexample on a visual display unit 15. There is an input means 16, e.g. anappropriate keyboard, connected to the control system 12, which enablesan operator of the device to interactively adjust the flip angle and theRF phase of the drive pulse as well as the duration of the recoveryperiod in order to optimize contrast and signal-to-noise.

1. Method for magnetic resonance imaging of at least a portion of a bodyplaced in a stationary and substantially homogeneous main magneticfield, the method comprising the following steps: a) subjecting saidportion of a body to a sequence of at least one RF pulse and at leastone magnetic field gradient pulse, thereby generating one or more spinecho signals in said portion; b) measuring said spin echo signals forreconstructing an image from said signals; c) generating an additionalspin echo by subjecting said portion to at least one further refocusingRF pulse and/or magnetic field gradient pulse; d) subjecting saidportion to a RF drive pulse at the time of said additional spin echo; e)repeating steps a) to d) with the pulse sequence of step a) beginningafter a recovery period; wherein the phase of said RF drive pulse isselected such that nuclear magnetization at the time of said additionalspin echo is transformed into negative longitudinal magnetization. 2.Method for magnetic resonance imaging according to claim 1, wherein thesequence of step a) begins with a substantially 90° RF pulse.
 3. Methodfor magnetic resonance imaging according to claim 1, wherein the drivepulse of step d) is a substantially 90° RF pulse.
 4. Method for magneticresonance imaging according to claim 1, wherein the sequence of step a)begins with a RF pulse, which has substantially the same flip angle asthe drive pulse of step d), both the initial RF pulse of step a) and thedrive pulse of step d) also having substantially the same RF phase. 5.Method for magnetic resonance imaging according to claim 4, wherein boththe initial RF pulse of step a) and the drive pulse of step d) have aflip angle of about 90°.
 6. Method for magnetic resonance imagingaccording to claim 1, wherein the pulse sequence of step a) is a turbospin echo (TSE) sequence beginning with a substantially 90° X-pulsefollowed by a series of 180° Y-pulses, wherein said drive pulse is againa substantially 90° X-pulse.
 7. Method for magnetic resonance imagingaccording to claim 1, wherein the pulse sequence of step a) is a RARE(rapid acquisition by repeated echoes) sequence, an EPI (echo planarimaging) sequence, or a GRASE (gradient and spin echo) sequence. 8.Method for magnetic resonance imaging according to claim 1, wherein foroptimizing the image contrast between tissues with different values ofT₁, the duration of said recovery period is selected such that it islarger than the shortest T₁.
 9. Device for magnetic resonance imaging ofa body placed in a stationary and substantially homogeneous mainmagnetic field, the device comprising means for establishing the mainmagnetic field, means for generating gradient magnetic fieldssuperimposed upon the main magnetic field, means for radiating RF pulsestowards the body, control means for controlling the generation of thegradient magnetic fields and the RF pulses, means for receiving andsampling magnetic resonance signals generated by sequences of RF pulsesand switched gradient magnetic fields, and reconstruction means forforming an image from said signal samples, wherein the control meanscomprises means for performing an imaging procedure according to themethod of claim
 1. 10. Device for magnetic resonance imaging accordingto claim 9, comprising an input means for enabling an operator of thedevice to interactively adjust the flip angle and the RF phase of thedrive pulse of step d) as well as the duration of the recovery period ofstep e).